After coming to Manuel Bursa, the son of the Serbian Emperor Yoannes, who accompanied Lightning Bayezit in the Karaman Expedition, he went to Istanbul unauthorizedly. Upon this incident, Lightning Bayezit canceled the previously planned Hungary expedition and decided to siege Istanbul, thinking that this was a secret purpose. Istanbul was besieged by land and sea (1391). The siege was a blockade because it was not big and strong. The siege was abolished as the Hungarians entered Turkish territory. This siege is the first siege of Istanbul made by the Ottomans. Hungarians who were not idle entered the Ottoman lands from the north. The Turkish athletes were sent to them, the Hungarian Army under the command of King Sigismund (1392). When returning from the Danube-Wallachian Campaign, Thessaloniki and its environs joined the Ottoman lands (1394). Lightning Bayezit surrounded Istanbul for the second time in 1395. But when it was reported that the Crusaders were moving, this siege was lifted before it succeeded, just like the first.
There was a very bloody battle in the Nigbolu war in 1396. The Crusaders have suffered a great deterioration in this conflict. At the end of the war, the Crusaders took over the Ottoman State. The Bulgarian Kingdom was abolished and the influx of Hungarians was made. The crusader world could not dare to walk on the Turks for half a century. After this war, Lightning Bayezit was given the title of "Sultan-i iklim-i Rum", "Anatolian Sultan" by Abbasi Caliphate. After the Battle of Nigbolu, Istanbul was besieged for the third time. Anadoluhisari, which had already begun to be built, was completed during this siege. A strong naval force and the absence of large balls prevented the conquest. For this reason, Lightning Bayezit tried to develop Turkish Maritime. Lightning kept Istanbul under siege and thought the city would surrender. However, when the Timorese danger emerged, a treaty with Byzantium was made and the siege removed. With this treaty, a mosque, an Islamic Court and a Turkish neighborhood were established in Istanbul Sirkeci. The annual tribute has been increased. The same year, the expedition to Greece and Mora was held.
Timur began operations in order to rebuild the Chengiz Empire. He took Iran and organized expeditions to India. Emirates of Azerbaijan and Baghdad took refuge in Bayezit. Thymus Emirs wanted to return, but Lightning Bayezit rejected it and because of this, Timur and Lightning Bayezit were opened. Timur, who entered Anatolia and plundered Sivas, continued to advance in Anatolia with the army of elected soldiers. The Ottoman Army was also active. Two armies were compared in Rod Plain in Ankara. The Black Tatars, one of the forces of Lightning in the Ankara battle, led to the dissolution of the Ottoman army on the side of Timur (July 20, 1402). Lightning Bayezit, Timur'a fell captive. This war caused the Ottoman State to pause for 50 years. The Anatolian Turkish unity broke up and the principalities in Anatolia reappeared and strengthened. Confusion began in the Ottoman Empire, which was a headless state. In four different regions of the Ottoman Empire, four separate states were proclaimed by princes. Bursa, Iznik and Izmit were looted and burned by Timur, Izmir was occupied. This period of powerlessness and throne struggles, which lasted from 1402 to 1413, was called the Fetret Period.